1. Field of the Invention
This invention relates to a laser beam recording system, and more particularly to an improvement in a laser beam recording system in which a video clock signal is generated in synchronization with a photoelectric pulse signal obtained through a linear encoder scanned with a laser beam deflected by a rotating polygonal mirror.
2. Description of the Prior Art
In a laser beam recording system, a video signal is generated by a character generator upon input of a video clock signal into the system and the video signal controls a light modulator which conducts amplitude modulation of a laser beam. The amplitude-modulated laser beam is deflected by a rotating polygonal mirror to form rasters on a photosensitive material and record characters in the form of a dot pattern.
Since it is substantially difficult to rotate a polygonal mirror at a precisely constant speed and there is a speed fluctuation in practice, the position of the dots is displaced and the dot pattern or the form of the recorded characters is deformed. Further, since the pitch of the dots is made larger in the marginal portions with respect to the central portion when the polygonal rotating mirror is used, it is difficult to record characters in a good form.
In view of the above mentioned difficulties of the conventional laser beam recording system, it has been known to use a linear encoder having a number of parallel slits arranged in the scanning direction scanned with a read-out beam prepared in addition to a recording beam and input the photoelectric pulse obtained through the linear encoder into a character generator as a video clock signal so that the dots are recorded at correct positions in view of the scanning position of the rotating polygonal mirror according to which a video signal is generated, as disclosed for instance in U.S. Pat. No. 3,389,403.
The above-described system using a linear encoder is, however, desirable only in case that the number of dots on the scan lines is small. When the number of dots is more than 2000, there are problems as follows. In the case where the resolution is so high, the pitch of the slits on the linear encoder is too small to enable the easy production of the linear encoder. Further, when the pitch of the slits is small, the photoelectric pulse signal is generated even by the dust or scratches on the slits, which results in reading mistakes through the linear encoder and mis-recording of the dot pattern.